1. Field of the Invention
The present invention relates to a high power pulsed light generation device that emits light obtained by amplifying a pulsed light emitted from a pulsed light source in an optical amplifier using a semiconductor laser as a pumping light source of the optical amplifier, and particularly relates to a high power pulsed light generation device which is suitable for laser processing.
2. Description of the Related Art
In general, as a high power pulsed light generation device having a function of pulsed light amplification, there are many devices, using a semiconductor laser (laser diode: LD) of a fiber output as a pumping light source of an optical amplifier, which are provided with an optical oscillator used as a pulsed light source and the optical amplifier that amplifies a pulsed light output from the optical oscillator.
In addition, in an LD for pumping of an optical amplifier (hereinafter, referred to as a pump LD), in many cases, there is provided a shunt type constant current driving circuit which is a highly precise resistor having a small resistance value, and the output of the pump LD is normally controlled to be constant by attached drivers.
However, in the circuit driving such a pump LD, an output current is optimized to be constant for a time period from 1 sec to 1 hr.
Accordingly, response time of the driving circuit is designed to be 1 msec or greater in many cases and it is difficult to turn on and turn off output light emitted from the pump LD at high speed.
Meanwhile, in order to facilitate reduced losses in a device in which a power circuit or a power supply circuit uses a constant voltage circuit, a constant voltage circuit applying a switching element which is enabled to perform pulse driving of the pump LD has been used.
As disclosed in prior art documents, a method of controlling an output waveform of a pump LD to be pulsed by performing the pulse driving of a driving current has also been known (for example, refer to U.S. Pat. Nos. 5,325,383 and 5,283,794).
Further, in the high power pulsed light generation device, in order to efficiently amplify a pulsed light (hereinafter, referred to as an oscillation pulsed light) emitted from an optical oscillator, there are techniques that perform the pulse driving of a pump LD disclosed in, for example, U.S. Pat. Nos. 5,867,305, 5,933,271, and 6,081,369.
In addition, a technology to improve a gain of an optical amplifier by changing the waveform of a driving current value of the pump LD to be a pulsed waveform and synchronizing the signal frequency of a pulsed laser light source with the frequency of a pulse output of the pump LD has also been known.
However, in the practical laser process, generally, an operation in which the high power pulsed light generation device continuously emits the pulsed light for processing only for a certain time; and temporarily stops the output of the pulsed light; and then, emits the pulsed light again, is repeated.
Here, a state where the output pulsed light is continuously emitted from the high power pulsed light generation device is referred to as an ON state and a state where the emission of the output pulsed light from the high power pulsed light generation device stops is referred to as an OFF state.
The change between the ON and OFF states of the high power pulsed light generation device is generally controlled by turning on and off a pulsed driving current of a pump LDs (pump LD) that generates pulsed light for pumping the optical amplifier (hereinafter, referred to as pulsed light for pumping).
Accordingly, the high power pulsed light generation device is in an OFF state when the pulsed driving current does not flow through the pump LDs and the high power pulsed light generation device is in an ON state when the pulsed driving current flows through the pump LDs.
It is also noted that the pulsed light for pumping is synchronized with the pulsed driving current, a pulse width of each pulse of the pulsed driving current of the pulsed light for pumping is generally constant, and in general, a pulse width of the pulsed light for pumping is sufficiently set longer than a pulse width of the output pulsed light.
In a case where such a high power pulsed light generation device of the pulse driving is used, just after the state of the high power pulsed light generation device is changed from the OFF state to the ON state, the gain of the optical amplifier for each pulse in the high power pulsed light generation device gradually increases from lower value to stable and higher value.
In addition, when increasing the gain of the optical amplifier for the pulse immediately after the change to the ON state in order to shorten the time until the gain reaches the stable state, the gain for the pulse may become excessive and overshot.
As stated above, there has been a problem that the gain of the optical amplifier for each pulse is not stable immediately after changing the state of the high power pulsed light generation device from the OFF state to the ON state, and as a result, the power of each individual pulse, which is output from the high power pulsed light generation device, is not stable.
Further, as stated above, a period until the power of the output pulsed light is stabilized after changing the state of the high power pulsed light generation device from the OFF state to the ON state is referred to as a transient period in the present invention, and a phenomenon causing fluctuation of the power of the output pulsed light for each pulse within the transient period is referred to as a transient phenomenon.
In particular, when using the high power pulsed light generation device as a light source of a laser processing device, the fluctuation of the power of the output pulsed light in the above-described transient period is a factor for enlarging the variation of processing properties such as a processing quality or the degree of processing in the laser processing.
Accordingly, in the transient period, it is necessary to hold the device on standby without performing the laser processing.
Such a standby period deteriorates operation efficiency of the laser processing device.
However, it has not been considered in any Patent documents described above to suppress the fluctuation of peak power of the output pulsed light due to the transient phenomenon when changing the state of the high power pulsed light generation device from the OFF state to the ON state.
Accordingly, in the high power pulsed light generation device of the conventional art, it is difficult to obtain an output pulsed light where the peak power is even immediately after the change when changing the state of the high power pulsed light generation device from the OFF state to the ON state.
Therefore, it has been difficult to avoid the deterioration of the operation efficiency of the high power pulsed light generation device due to the standby period as described above.
FIG. 7 shows a time chart of, a pulsed pumping light of an optical amplifier and, an output pulsed light (signal) emitted from the optical amplifier and, a gain value of the optical amplifier and, a master clock signal, in the high power pulsed light generation device of the conventional art, when all the data are started from a timing of start trigger of output pulsed light for high power pulsed light generation device.
As shown in FIG. 7, regarding the gain of the optical amplifier, the peak power of first pulse of the output pulsed light immediately after changing the state of a high power pulsed light generation device from an OFF state to an ON state is lower than peak power (pulse height) in a stable ON state, and then, the peak power of first pulse gradually increases as the pulse is emitted and the power reaches the peak power in a stable state after emitting the pulse several times.
Accordingly, immediately after changing the state of the high power pulsed light generation device from the OFF state to the ON state, a lower power pulse than that of the output pulsed light emitted in a stable state is emitted, and then, the power fluctuates for each pulse until the power reaches the stable state.
The cause that such a phenomenon occurs is considered as follows.
The phenomenon occurs because the gain of the optical amplifier in an OFF state is lower than the minimum gain in a stable ON state (hereinafter, referred to as a stable state).
Further, it is considered that the phenomenon occurs because the pulse width of the pulsed light for pumping output from the pump LD is controlled to be constant by setting the pulse width of the pulsed driving current with respect to the pump LD to be constant at all times.
In a case of FIG. 7, the pulse width of the pulsed light for pumping emitted from the pump LD as described above is controlled to be constant. However, a timing of changing the state of the high power pulsed light generation device from the OFF state to the ON state (timing of an output start trigger) is deviated from a timing of starting the pulse of the master clock signal.
Accordingly, the width of the first pulse of the pulsed light for pumping immediately after the change to the ON state becomes small.
Therefore, the gain of the optical amplifier also becomes extremely small in the first pulse.
As stated above, in the high power pulsed light generation device of the conventional art, the peak power of the output pulsed light fluctuates when changing the state of the high power pulsed light generation device from the OFF state to the ON state.
Accordingly, in case the high power pulsed light generation device of the conventional art is used for laser processing, working efficiency will be reduced. Because, when the peak power of the high power pulsed light generation device is fluctuating just after turning from the OFF state to ON state, the processing must be stopped and wait until the peak power become stable.
The present invention is devised in view of the foregoing problem, and provides a high power pulsed light generation device that can suppress the power fluctuation of the output pulsed light of the high power pulsed light generation device. In this case, the high power pulsed light generation device can stabilize the power of the output pulsed light for each pulse in an early stage, for example, even when changing the state of the high power pulsed light generation device from the OFF state to the ON state, and can prevent the above-described transient phenomenon from occurring.